1. Field of the Invention
The invention relates to a method and a device for matching the phase between the pixel clock of a graphics card and the sampling clock of a flat-panel display with an analog interface in a system comprising flat-panel display, graphics card and computer.
2. Background Art
Flat-panel displays with an analog interface must be adapted to the graphics card of the connected computer. If phase or sampling frequency is incorrectly adjusted, the image appears fuzzy and contains interferences.
Whereas the values for image location, or in other words right-left and top-bottom adjustment, and for sampling frequency can be defined as preadjusted values in the case of standard modes, this is not possible for the phase, since the phase depends on the graphics card used and also on the video circuit.
Prior art flat-panel displays are usually provided with a microprocessor, which is responsible for general control of the flat-panel display. This microprocessor is configured such that it can also recognize the video mode adjusted on the computer. If the mode has already been adjusted at the factory or by the user, the flat-panel display is operated with the stored adjustments for image location, sampling frequency and phase. On the other hand, if the mode is one which has not yet been implemented in the microprocessor of the flat-panel display, standard values are assumed for image location, sampling frequency and phase. These standard values are not satisfactory in all cases.
The adjustment of the sampling clock and of the phase have a direct effect on image quality. An optimal sampling frequency is achieved when the sampling of all pixels, in one line of a video signal, for example, takes place in a stable or characteristic region of these pixels, such as at the center of each pixel. Data conversion then yields optimal results. The displayed image does not contain any interferences, and is stable. In other words, the optimal sampling frequency is equal to the pixel frequency. If an incorrect sampling frequency has been adjusted, for example if the sampling clock is too fast compared with the pixel clock, the pixels are sampled at first in the permissible region, or in other words at the midpoint between two edges, but the subsequent pixels are sampled progressively more toward one edge, until even the region between two pixels is sampled, which obviously leads to unsatisfactory image quality. Incorrect sampling values are derived from the region in which the pixels are not sampled in an optimal, characteristic region. The image then exhibits strong vertical interference. The number of regions with vertical interference that are visible on the monitor increases as the difference between the frequencies of the sampling clock and the pixel clock becomes larger.
Even in the cases in which the sampling clock is identical to the pixel clock, however, the image quality can suffer if the phase has not been adjusted correctly. The reason is that sampling takes place in a pixel region that is not ideally suitable for sampling, for example too close to the leading or trailing edge of a pixel. This problem can be solved by shifting the phase, or in other words the sampling instant, as the whole until sampling takes place in a characteristic or permissible region of the pixels. If the phase has not been adjusted correctly, the image quality is impaired by noise signals over the entire monitor.
For this reason the users are usually instructed in the manuals and by notices on the packing to perform the necessary adjustment of the phase themselves, but this is unsatisfactory, especially for less experienced users.
Flat-panel displays with analog interfaces, in which phase adjustment is automatically performed, are already known. For such automatic phase-position adjustment, special test patterns with alternating white and black image spots are necessary, and the test pattern must be displayed by the graphics card. This has the disadvantage that software must be installed and started on the computer, and furthermore that this software must be available for all common operating systems.
From German Patent 3914249 A1 there is known a method for recovery from an input signal generated with an unknown clock, wherein the input signal is digitized with a reference clock at different phase positions. The difference between the clock frequency of the input signal and the reference clock is determined from the variation of the phase position (input signal relative to reference clock), and the frequency of the reference clock is corrected accordingly.
A signal-processing method for an analog image signal is described in German Patent 19751719 A1. Therein the analog image signal is obtained from a computing unit, in which the signal has been digitally generated according to a graphics standard such as EGA or VGA and then converted to analog form. The method comprises subjecting the analog image signal to analog-to-digital conversion with a first selected sampling frequency, after which the sampled image is examined for image perturbations, in order to determine a corrected sampling frequency. Further measures relate to determination of the optimal sampling phase and determination of the exact position of the active image relative to the horizontal or vertical synchronization pulses.
In this regard, the object of the invention is to provide a method and a device for matching the phase in flat-panel displays, whereby automatic phase adjustment is possible without the use of test patterns.
To achieve this object, an inventive method is characterized in that the rising edge of a video pulse of a sufficiently bright image spot is determined, in that the falling edge of the video pulse is determined at a sufficiently bright image spot and in that the phase is adjusted such that the sampling instant is situated approximately at the midpoint between the rising and falling edges of a video pulse.
To achieve the said object, an inventive method is further characterized in that the rising edge of a video pulse of a sufficiently bright image spot is determined, and in that the phase is adjusted such that the sampling instant is shifted by approximately half the width of an image spot toward the center of the pixel.
To achieve the said object, an inventive method is further characterized in that the falling edge of the video pulse is determined at a sufficiently bright image spot, and in that the phase is adjusted such that the sampling instant is shifted by approximately approximately half the width of an image spot toward the center of the pixel.
Whereas the image-location and sampling frequencies can be determined and correspondingly adjusted relatively simply by an algorithm, the phase position is more difficult to determine. The three said inventive methods are simple and satisfactory methods for adjusting the phases.
An advantageous embodiment of the inventive method, wherein the image area and image spots are arrayed on the flat-panel display in rows and columns between a back-porch region and a front-porch region, is characterized in that an image spot in the first image column close to the back-porch region is chosen as the sufficiently bright image spot for determination of the rising edge and an image spot in the first image column close to the front-porch region is chosen as the sufficiently bright image spot for determination of the falling edge. The method can be performed particularly well if the most pronounced possible edges are evaluated or if regions or spots disposed next to one another have very different brightness. Thus a spot in the first or last image column is particularly suitable, since it completely satisfies the required conditions in combination with the front-porch or back-porch region respectively, and can be found with relatively little difficulty.
An advantageous embodiment of the inventive method is characterized in that the brightness of a plurality of image spots of the first or last image column is measured, and the image spot with the greatest or adequate brightness in the first or last image column is chosen for determination of the rising or falling edge respectively of the video pulse. In this way it is ensured that image spots with sufficiently pronounced edges are used for the measurement.
An advantageous embodiment of the inventive method is characterized in that the image spots (nxc3x97k) are first measured with n=1, 2, . . . N and k=constant, such as 10, and in that, if no adequately bright image spot was found, the image spots (n+m)xc3x97k are measured with m=1, 2, . . . N, until a sufficiently bright image spot is found. Thereby a search for suitable image spots is performed efficiently and in the shortest time.
An advantageous embodiment of the inventive method is characterized in that, for determination of the amplitude value of the image spot, the phase is shifted until the measured amplitude values no longer change significantly, and in that the amplitude value then determined is further processed.
Alternatively, an advantageous embodiment of the inventive method is characterized in that the phase used for determination of the amplitude value is advanced sufficiently that the measured amplitude values are smaller than a predetermined limit value, for example smaller than 50% of the amplitude value, in that the phase is delayed by half the width of a spot, and in that the amplitude value then measured is further processed.
The last two of the foregoing embodiments of the inventive method are simple solutions in order to determine the brightness of the image spot as a prerequisite for determination of the position of the rising and falling edge of the image spot.
A further advantageous embodiment of the invention is characterized in that, for determination of the rising edge, the phase is shifted sufficiently toward the back-porch region that the measured amplitude value is reduced to a predetermined percentage, for example 50%, of the previously determined amplitude value, and in that this value of the phase is stored temporarily as the position of the rising edge. Yet another advantageous embodiment of the invention is characterized in that, for determination of the falling edge, the phase is shifted sufficiently toward the front-porch region that the measured amplitude value is reduced to a predetermined percentage, for example 50%, of the previously determined amplitude value, and in that this value of the phase is stored temporarily as the position of the falling edge. In this way the rising and falling edges of two image spots are determined in simple manner, and the phase can then be adjusted such that it is located between the rising and falling edges at approximately the center of an image spot.
A further advantageous embodiment of the invention is characterized in that the phase or sampling instant is delayed relative to the midpoint between the rising and falling edges by a predetermined amount, for example 10% of the width of the image spot. This is advantageous in particular for rapid video signals with overshoots, since it prevents sampling from taking place in the region of the overshoot.
To achieve the object cited hereinabove, the device for matching the phase between the pixel clock of a graphics card and the sampling clock of a flat-panel display having an analog interface in a system comprising a flat-panel display, graphics card and computer, is characterized by a device that determines the rising edge of a video pulse of a sufficiently bright image spot, a device that determines the falling edge of the video pulse at a sufficiently bright image spot, and an adjusting device with which the phase is adjusted such that the sampling instant is located at approximately the midpoint between the rising and the falling edges of a video pulse.
A further advantageous embodiment of the inventive device is characterized by a device which determines the rising edge of a video pulse of a sufficiently bright image spot, a device that determines the falling edge of the video pulse at a sufficiently bright image spot, and an adjusting device with which the phase is adjusted such that the sampling instant is located at approximately the midpoint between the rising and the falling edges of a video pulse.
Further advantageous embodiments of the inventive method and of the inventive device are evident from the remaining dependent claims.